1. Field of the Invention
The present invention relates to an arrangement and to a method for generating a reference impulse. The present invention also relates to a position measuring device having a corresponding arrangement for generating a reference impulse. A reference impulse is needed in incremental position measuring devices for defining a reference point for the position measurement.
2. Description of the Related Art
Incremental position measuring devices are known from many published references. They serve above all to determine relative displacements or the absolute position of machine parts, for instance in machine tools or measuring machines.
Such position measuring devices essentially include a scale graduation, on which one or more division tracks are applied, and a scanning unit, which scans the division tracks and converts changes in distance or angle into electrical signals. In length measuring instruments, the scale graduation is embodied for instance as a scale. If a length measuring instrument is used for measuring relative machine movements, the scale can be mounted in stationary fashion on the machine, while the scanning unit is secured to a movable part of the machine, such as a tool carriage, whose relative motion is to be measured relative to the machine. When the tool carriage is moved, the scanning unit moves in a scanning plane in the measurement direction parallel to the division tracks disposed on the scale so as to scan the division tracks, wherein such division tracks are located in a scale plane, and scans the division tracks. In the process, position signals are generated, which indicate the relative change in position of the scanning unit relative to the scale.
In incremental position measuring devices, the division tracks include code elements disposed uniformly one after the other in the measuring direction. Such division tracks are also called incremental tracks. From scanning one incremental track, usually either two position signals phase-offset from one another by 90°, or four position signals having have the phase relationships 0°, 90°, 180°, and 270°, are obtained. In addition, position measuring devices are also known which generate three position signals having the phase relationships 0°, 120° and 240°. Given a uniform motion of the scanning unit relative to the scale graduation, these position signals are largely sinusoidal. The position determination is effected by counting elapsed signal periods. Ascertaining an absolute position from the measurement of relative positions requires the creation of a reference point. This purpose is served by at least one so-called reference mark, which is disposed for instance on a reference track next to the incremental division track and which is likewise read off by the scanning unit. When the reference mark is scanned, an analog current or voltage pulse is created, from which in further processing a rectangular digital signal, a so-called reference impulse, is generated.
In the final analysis, the reference impulse is used in a downstream electronic unit for determining the reference position that is used as a reference point for the position measurement. The processing of the analog current or voltage pulse to make the reference impulse can be done for instance by a comparator, in which the analog input signal is compared with a defined switching threshold, and the output of which is switched accordingly.
The central demands made of the reference impulse are that the reference impulse has a defined location and width related to the position signals. If these demands are not met mistakes in ascertaining the reference point can occur in the downstream electronic unit in the logical linkage of the reference impulse with the position signals. For example, if the reference impulse is too narrow, it can happen that the reference impulse will fail to be detected; if it is too wide, the reference impulse may under some circumstances be detected at two different points. It is also especially problematic that in both cases, the location of the reference impulse relative to the position signals can be decisive as to whether the reference impulse is correctly detected, or not. Thus, in limited situations, even a slight change in the location of the reference impulse, for instance caused by temperature fluctuations, can be decisive as to the correct detection of the reference impulse. Both a missing reference impulse and a reference impulse detected twice, however, lead to an error situation that can mean failure of the machine on which the position measuring device is operated.
Until now, the location of the reference impulse has predominantly been adjusted by complicated mechanical calibration of the scanning unit once the position measuring device has been mounted on the target application, such as a machine tool or a wafer scanning unit. In some scanning principles, the analog reference impulse can be shifted relative to the incremental division track as a result of rotation of the scanning unit. The width of the reference impulse can be adjusted by varying the comparator threshold, that is, the switching threshold that the analog current or voltage pulse must exceed or undershoot in order to generate the turn-on or turn-off edge of the reference impulse. Alternatively, a positive or negative offset can also be superimposed on the analog reference impulse. The higher the resolution of the position measuring device, the greater the expense and complexity of this mechanical and electrical calibration. This method is of virtually no use with division periods in the range of a few micrometers. A further factor is that at these resolutions, thermal expansions, the effects of contamination, and so forth can generate effects that attain orders of magnitude, which affect the location and width of the digital reference impulse to a critical extent, even if exact mechanical calibration of the scanning unit is done.